1. Field of the Invention
The present invention relates to a semiconductor pressure sensor equipped with a diaphragm that is moved in response to the pressure of a fluid to be measured for detecting a pressure such as, for example, the pressure in an intake manifold of an internal combustion engine, the internal pressure of a tank, etc.
2. Description of the Related Art
In the past, there has been known a semiconductor pressure sensor that has a diaphragm formed on one surface of a silicon substrate with a piezoresistive element formed on the diaphragm.
In this case, the diaphragm is caused to distort under the pressure of a fluid to be measured to change the resistance value of the piezoresistive element, whereby the pressure of the fluid to be measured can be detected by outputting an amount of change in the resistance value as an electric signal.
However, the fluid to be measured, which is an object for pressure measurement, generally has an electric charge whether it is a gas or a liquid, so when the fluid to be measured comes into contact with the diaphragm upon measurement of the pressure, the charged object is placed into contact with the diaphragm, too, whereby the resistance value of the piezoresistive element is caused to change under the influence of an electric field generated from this charged object, thus making an electric signal output from the piezoresistive element unstable.
To cope with such a problem, there has been known a semiconductor pressure sensor equipped with a diaphragm whose surface is covered with an insulating film (see a first patent document: Japanese patent application laid-open No. S63-250865 (FIG. 13)).
In this case, a distance between a piezoresistive element and a charged object is increased by the formation of the insulating film, whereby the influence on the piezoresistive element due to an electric field generated from the charged object can be reduced.
In addition, as another measure to deal with the above problem, there has also been known a semiconductor pressure sensor equipped with a diaphragm whose surface is covered with a grounded shield film (see a second patent document: Japanese patent application laid-open No. H11-281509 (FIG. 7)).
In this case, if electric charge is attached to the surface of the shield film on the diaphragm, the electric charge flows out to the outside due to the grounding of the shield film, so that the influence of an electric field from the charged object on the piezoresistive element can be prevented.
However, the above-mentioned first patent document involves the following problem. That is, in order to suppress the influence of the electric field due to the charged object to a low level, it is necessary to increase the thickness of the insulating film thereby to enlarge the distance between the piezoresistive element and the charged object. This increases the thickness of the diaphragm itself, so the amount of displacement of the diaphragm, which is caused to displace in accordance with the pressure of the fluid to be measured, becomes small, thus resulting in that a satisfactory level of sensitivity can not be obtained.
Also, in the case of the above-mentioned second patent document, there is the following problem. That is, when the silicon substrate has a reference potential, an electric field is necessarily generated between the shield film, which is zero potential, and the silicon substrate due to a potential difference therebetween, so the piezoresistive element is always subject to the electric field irrespective of the presence or absence of the charged object, as a result of which if the reference potential varies, the electric signal output from the piezoresistive element will also vary in accordance with the reference potential variation, thus making measured values unstable.